In the field of manufacturing semiconductor wafers, vacuum pumps have conventionally been used. Use is made of, for example, a screw pump as the vacuum pump. The screw pump is disclosed, for example, in the following non-patent literature 1 as a screw type pump.
Non-Patent Literature 1: “Physics Dictionary” compiled by Physics Dictionary Editorial Board, Baifukan, Revised Edition published May 20, 1992, p. 1019
Generally, the screw pump comprises a pair of screw rotors having a first screw rotor with a plurality of helical land portions (a male rotor with convex thread ridges) and a second screw rotor with a plurality of helical groove portions (a female rotor with concave thread grooves) and adapted to rotate about two axes substantially parallel to each other while engagement with each other, and a casing receiving therein the pair of screw rotors and having an inlet port and a discharge port. Herein, since the first screw rotor (the male rotor with the convex thread ridges) has not only the plurality of helical land portions but also a plurality of helical groove portions, it can also be said that the first screw rotor has the plurality of helical land portions and the plurality of helical groove portions. Likewise, since the second screw rotor (the female rotor with the concave thread grooves) has not only the plurality of helical groove portions but also a plurality of helical land portions, it can also be said that the second screw rotor also has the plurality of helical land portions and the plurality of helical groove portions. Further, a pair of shafts supporting the pair of screw rotors are provided with a pair of bearings and a pair of shaft seal members.
In the conventional screw pump, ball bearings are generally used as the bearings. Therefore, seal mechanisms such as oil seals or mechanical seals are added between the screws and the ball bearings. However, since it is not possible to completely prevent leakage of ball bearing oil to the screw side and further since a large amount of gas is introduced to the seal portions, there has been a technical problem that, in case of being used as a vacuum pump in a processing step (plasma etching, reduced-pressure vapor phase epitaxy) that emits a toxic gas, a corrosive gas, or the like in a pressure-reduced state when, for example, manufacturing semiconductor devices, the gas contacts the ball bearings so that the bearings are corroded and reaction product is accumulated on the ball bearings to impede smooth operation.
Further, there has been a technical problem that since the gas introducing amount is large, a huge cost is required for separating and recovering an expensive gas such as Kr or Xe used in the processing step.